KrF positive resist compositions, which have been proposed as ideal resist materials for exposure methods using a KrF excimer laser, typically utilize a polyhydroxystyrene-based resin in which a portion of the hydroxyl groups have been protected with acid dissociable dissolution inhibiting groups as the base resin. As the acid dissociable dissolution inhibiting group, so-called acetal groups including chain-like ether groups typified by 1-ethoxyethyl groups, cyclic ether groups typified by tetrahydropyranyl groups, tertiary alkyl groups typified by tert-butyl groups, and tertiary alkoxycarbonyl groups typified by tert-butoxycarbonyl groups are the most commonly used. See Patent References 1-3 below.
[Patent Reference 1]
Japanese Unexamined Patent Application, First Publication No. 2000-227658
[Patent Reference 2]
Japanese Unexamined Patent Application, First Publication No. 2000-188250
[Patent Reference 3]
Japanese Unexamined Patent Application, First Publication No. 2000-356850
In a resist composition comprising this type of polyhydroxystyrene-based resin as the base resin, the proportion of hydroxyl groups protected with the acid dissociable dissolution inhibiting groups is preferably high in order to ensure a high level of resolution. However, if the protection ratio provided by the acid dissociable dissolution inhibiting groups is too high, then the resist pattern tends to be prone to pattern defects following developing (hereafter also referred to as developing defects).
In the patent reference 1, a positive resist composition is disclosed which uses a copolymer comprising polymerization units derived from a hydroxystyrene monomer, polymerization units derived from 3-hydroxy-1-adamantyl methacrylate, and groups that are unstable in the presence of acid, and this copolymer becomes alkali soluble under the action of acid. However, a positive resist composition using this copolymer is particularly prone to microbridge-type developing defects.